I just got done making this suggestion to someone on facebook.
Take an old (or new) flip up scope cap like a buttler creek, that's junk anyway, and drill a hole in the middle with either a drill or dremel.
Start small and keep checking till you can see what you want to see with it.
I usually stay around 1/2" but added a few extra holes for shits and giggles. The ONLY time I use it is coyote hunting
The nylon stretched over the objective is a fine option as well, but this only reduces light, it doesn't reduce clarity like the nylon does.
The size of the hole will depend a lot on the zoom of your optic and if you use a sunshade or not.
If you REALLY wanna take your time, you could grid it off and make your own ARD...but this works just as well.